There has been increasing interest in electroluminescence because of its promise for providing light cells useful in flat panel displays.
It is known that electrons allowed to drift in a high-field region in a host lattice that includes optically active ions or centers can obtain sufficiently high kinetic energies to excite the optically active ions or centers. This mechanism has been described as impact excitation and been used to explain luminescence in rare-earth and transition metal fluoride molecules in II-VI compounds, such as zinc sulphide. For impact excitation to be the dominant mechanism in such cases, it is important that the exciting electrons not be allowed to gain kinetic energies sufficient for impact ionization of either the optically active centers or the host lattice. This can be best accomplished when the total cross section for impact ionization is less than the cross section for impact excitation. Various optically active centers meet this requirement. Of particular interest is europium.
Europium ions are known to be especially attractive as a source of electroluminescence, the europium ion radiating in the red when in a trivalent state and in the blue when in the divalent state, when placed in suitable local field configurations.
The use of optically active molecular centers is known to have important advantages. First, ions of appropriate valencies for optical activity can usually be introduced in large concentration for large outputs, not necessarily limited by chemical compatibility with the host lattice. Often the chemical and metallurgical compatibility aspect has been a major difficulty to introducing rare-earth ions in II-VI compounds as optically active centers. Moreover, the relatively large size of molecular centers desirably provides a relatively large cross section for inelastic scattering of the hot electrons. Additionally, the nonradiative decay of the excited states is minimized since the included molecules typically have a poor phonon impedance match with the host lattice. Most molecules useful as optically active centers have been fluorides since the fluorine compounds involve strong bonds to the optically active centers. Moreover, fluorine compounds usually sublime and retain their molecular form when heated, and this makes these compounds well adapted for use where molecules of the compound are to be introduced into the host lattice by codeposition in a evaporation process.
Generally in the past, europium has been used as an element of a binary fluorine compound. However unfortunately such compounds when heated tend to dissociate resulting in poor valency control, so that when one seeks to introduce molecules of such europium compounds as optically active centers into a host lattice by codeposition evaporation, the resultant is a host lattice in which the europium has sizable concentrations of both divalent and trivalent states. As a consequence the emitted light is a weak mix of blue and red light, and not an efficient source of a single color, as is often desirable, due to non-radiative interactions between the ions of the two valency states.
An object of this invention is to use more effectively molecules including europium as optically active centers as sources of luminescence of a single color. Such sources are needed if they are to be used in a multicolor display.